Adsorption-Induced Intramolecular Dipole: CorrelatingMolecular Conformation and Interface Electronic Structure

Norbert Koch,*,† Alexander Gerlach,‡ Steffen Duhm,† Hendrik Glowatzki,†

Georg Heimel,† Antje Vollmer,§ Yoichi Sakamoto,| Toshiyasu Suzuki,|

Jörg Zegenhagen,⊥ Jürgen P. Rabe,† and Frank Schreiber‡

Humboldt-UniVersität zu Berlin, Institut für Physik, Newtonstrasse 15, 12489 Berlin, Germany,UniVersität Tübingen, Institut für Angewandte Physik, 72076 Tübingen, Germany, Berliner

Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung m.b.H., 12489 Berlin, Germany,Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan, and European Synchrotron

Radiation Facility, BP 220, 38043 Grenoble Cedex 9, France

Received January 13, 2008; E-mail: norbert.koch@physik.hu-berlin.de

Abstract: The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules andCu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunnelingmicroscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distancesfor PEN and PFP differ strongly, that is, 2.34 Å for PEN versus 2.98 Å for PFP. An adsorption-inducednonplanar conformation of PFP is suggested by XSW (F atoms 0.1 Å above the carbon plane), whichcauses an intramolecular dipole of ∼0.5 D. These observations explain why the hole injection barriers atboth molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas themolecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results showthat the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionizationenergy of molecules is not always readily applicable.

Introduction

The magnitude of electron (hole) injection barriers (EIBs andHIBs) at organic semiconductor/metal interfaces is crucial forthe performance of organic (opto-) electronic devices.1,2 Con-sequently, considerable effort has been directed toward develop-ing a coherent picture of organic/metal interface energetics 1–6

and approaches to control the energy level alignment.7–10 Onestrategy believed to decrease the EIB (HIB) at organic/metalinterfaces is to increase (decrease) the ionization energy (IE)of the organic material by ∆IE. Such an increase of the molecular

IE can, e.g., efficiently be facilitated by fluorination substitution.Within the simple model of the Schottky-Mott limit, the EIB(HIB) of the molecular layer then decreases (increases) by theamount ∆IE for any metal contact. Because the invalidity ofthe Schottky-Mott limit (i.e., vacuum level alignment) fororganic/metal interfaces is well documented,3,4 the applicabilityof this simple model can be challenged. The actual value ofcharge injection barriers is governed by the type and strengthof organic/metal interaction, often accompanied by metal surfacecharge redistribution due to the adsorbed molecules and alsocharge transfer between metal and molecules. In addition, themolecular charge reorganization energy, charge polarization bysurrounding matter, and changes of molecular conformation(possibly inducing/changing intramolecular dipoles) stronglyinfluence charge injection barrier heights. This complex com-bination of several effects makes it practically impossible topredict injection barriers based on simple models. Therefore, itis necessary to draw on complementary information that yieldelectronic and structural properties of interfaces between anorganic molecule and a metal surface.

Using different experimental techniques we have studied theinterfaces formed between two prototypical organic semicon-ductor materials, that is, pentacene (PEN, C22H14) and perfluo-ropentacene (PFP, C22F14), and Cu(111). Both are widely studiedmaterials today, as they are used in organic field-effect transis-tors with high hole (PEN)11 and electron (PFP)12 mobility.

† Humboldt-Universität zu Berlin.‡ Universität Tübingen.§ Berliner Elektronenspeicherring-Gesellschaft für Synchrotronstrahlung

m.b.H.| Institute for Molecular Science.⊥ European Synchrotron Radiation Facility.

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Published on Web 05/14/2008

10.1021/ja800286k CCC: $40.75 2008 American Chemical Society7300 9 J. AM. CHEM. SOC. 2008, 130, 7300–7304

Employing ultraviolet and X-ray photoelectron spectroscopy(UPS, XPS), X-ray standing wave (XSW) measurements,scanning tunneling microscopy (STM), supported by quantumchemical modeling, allows to assess hole injection barriers,sample work function changes, strength of organic/metalinteraction, molecular order, and molecule-metal bondingdistances, molecular distortions, and distortion-induced intramo-lecular dipoles. This comparative study thus enables providinga comprehensive picture of interfacial phenomena for the twoarchetypal molecules.

Experimental Section

UPS and XPS experiments were performed with the endstationSurICat at beamline PM4 (BESSY),8 for XSW experiments we usedbeamline ID32 (ESRF),13 and STM measurements were done usingan Omicron VT-STM attached to a custom vacuum system at theHU-Berlin. In all cases, ultrahigh vacuum (UHV) conditions werepreserved during sample preparation and measurements (basepressure < 5 × 10-10 mbar), and a higher base pressure in theevaporation chamber of SurICat (5 × 10-9 mbar). Cu(111) crystalswere cleaned by repeated Ar-ion sputtering/annealing cycles. PEN(Sigma-Aldrich) and PFP14 were evaporated from resistively heatedpinhole sources, and the deposited mass-thickness was monitoredby quartz crystal microbalances. Density-functional theory (DFT)calculations on isolated, fully geometry-optimized PEN, and PFPmolecules were performed at the B3LYP/6-31G** level.15 Ad-ditionally, PFP was geometry-optimized under the constraints thatthe F atoms are located 0.1 Å above the plane of the C atoms. Theintramolecular dipole was then extracted from this model PFPconformation (as suggested by our experiments).

Results and Discussion

The C1s core-level of the PEN monolayer on Cu(111)[bottom curve in Figure 1a] is split by 0.65 eV into two

components, very similar to the case of PEN/Cu(110)16 andPEN/Cu(119),17 where strong organic/metal chemisorptioninvolving a hybridization of molecular orbitals and metal stateshas been suggested.18 The broad high binding energy componentrequired for adequate spectral fitting indicates pronouncedshakeup processes for PEN chemisorbed on Cu(111), that is,low-energy losses of emitted photoelectrons due to electron–holepair creation within the new hybrid molecule/metal density ofstates close to the Fermi level (EF).17,19 PEN multilayers yieldonly a single but asymmetric C1s peak (top curve in Figure 1a)due to a superposition from the chemically slightly inequivalentC atoms in pristine bulk PEN,20 whose binding energy differ-ences are too small to be resolved experimentally in the solidstate.17 The continuum-like shakeup processes occurring for thechemisorbed PEN monolayer are absent for the multilayerbecause no hybrid molecule/metal states close to EF exist inmultilayer PEN. Consequently, the binding energy of inequiva-lent carbon species in PEN is significantly altered due to thestrong interaction with the Cu(111) substrate. The C1s core-level of the PFP monolayer on Cu(111) also has two compo-nents, separated by 1.5 eV (bottom curve in Figure 1b). Thelow binding energy (BE) C1s component in this spectrum (at284.4 eV BE) is due to some residual carbon contamination,which adsorbed on the metal surface during PFP sublimationand sample transfer that was necessary for XPS and UPSmeasurements. In addition, a few PFP molecules (less than 5%of a monolayer) may have reacted on the substrate at defectsites/step edges, as suggested by STM in Figure 2a and discussedbelow. However, the vast majority of adsorbed PFP moleculesremains intact at room temperature, as only annealing to 150°C leads to a significantly changed C1s spectrum (center curve

(13) Gerlach, A.; Schreiber, F.; Sellner, S.; Dosch, H.; Vartanyants, I. A.;Cowie, B. C. C.; Lee, T. L.; Zegenhagen, J. Phys. ReV. B 2005, 71,205425.

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Figure 1. C1s core levels of (a) PEN monolayer (bottom) and multilayer (top) on Cu(111), and (b) PFP monolayer on Cu(111) (bottom) and on Au(111)(top), and PFP monolayer on Cu(111) after annealing to 150 °C (center curve), providing evidence for a temperature-induced reaction. All spectra weretaken at beamline PM4 (BESSY), the photon energy was 630 eV.

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in Figure 1b) indicating a strong chemical reaction with themetal substrate, and the appearance of more “pinned” PFPmolecules in STM (not shown). The two components of theC1s core-levels assigned to nonreacted PFP are explained bythe chemically highly inequivalent carbon atoms within amolecule, that is, 14 C atoms directly bound to F and 8 C atomsbound just to neighboring C atoms. The ratio of these two Cspecies was found experimentally (from the area of thecorresponding peaks) to be 1.78 ( 0.05, in good agreementwith the expected value of 1.75 from the molecular stoichiom-etry. Peak-splitting due to strong chemisorption can be ruledout, as both spectral shape and magnitude of peak splitting forthe monolayer on Cu(111) are virtually identical to thoseobtained for a PFP monolayer on Au(111) (top curve in Figure1b), for which a weak interaction was found.21

The significantly weaker adsorption of PFP compared to PENon Cu(111) is further evidenced by STM imaging. For sub-monolayer coverage, no stable images of laterally ordereddomains could be obtained (Figure 2a). Instead, we observed afew single molecules or small clusters on terraces, anddisordered larger clusters near Cu step-edges. Apparently, somePFP molecules reacted at step-edges and point-defects onterraces,22 contributing mainly to the low BE component at284.4 eV of the C1s spectrum. Most of the terraces appearfeatureless (at room temperature), indicating that the PFPsubmonolayer resembles a 2-dimensional liquid phase, wheremolecules can easily be moved laterally during imaging by theinteraction with the STM tip. A similar phenomenon has beenreported for PEN/Ag(111).23 Note that for PEN on Cu(100)terraces individual molecules of a disordered phase could beimaged at room temperature, due to strong chemisorption.24

Upon formation of a full PFP/Cu(111) monolayer a stableordered structure of lying molecules is formed, as depicted inFigure 2b.

Based on these findings, one could speculate on a smalleraverage C-Cu adsorption distance for the chemisorbed PEN

compared to the weakly bound PFP. In fact, this is confirmedand quantified by XSW measurements which probe the atomicpositions of the adsorbate relative to the substrate latticeplanes.13,25,26 The characteristic variation of the photoelectronyield in the X-ray interference field of the Bragg reflectionprovides different coherent positions (PH) for the atoms in thePEN and PFP molecules (Figure 3a). Converting this phaseinformation into distances and applying nondipole corrections,13

we find an average C-Cu bonding length of (2.34 ( 0.02) Åfor PEN and (2.98 ( 0.07) Å for PFP, that is, a huge differenceof 0.64 Å between the positions of the carbon cores of the twoadsorbates above the metal surface. Moreover, the XSW resultsindicate that the PFP molecule does not adsorb in its coplanarbulk conformation, as might be expected due to the weakmolecule-metal interaction. We find that the F atoms of PFP

(21) Koch, N.; Vollmer, A.; Duhm, S.; Sakamoto, Y.; Suzuki, T. AdV.Mater. 2007, 19, 112.

(22) PFP molecules on Cu(111) can be made to react strongly with themetal surface, however, only upon heating to over 150 °C.

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Figure 2. STM micrographs of (a) a liquid-like submonolayer of PFP onCu(111) including several reaction-induced pinning centers (arrows) (imag-ing parameters: –1.2 V, 0.5 nA), and (b) of an ordered monolayer PFP onCu(111) with submolecular resolution, unit-cell (containing one PFP)parameters: a ) (1.6 ( 0.1) nm, b ) (0.8 ( 0.1) nm, R ) (96 ( 2)°(imaging parameters: –0.8 V, 0.5 nA). Images were obtained at roomtemperature.

Figure 3. (a) X-ray standing wave scans obtained on submonolayers ofPEN and PFP on Cu(111). The symbols represent the photoelectron yield(circles) and reflectivity (triangles) data measured on both adsorbate systems.The solid lines show least-squares fits based on dynamical diffraction theorywhich provide the coherent position PH and coherent fraction fH for eachelement [13]. The characteristic shapes of these curves reveal the differentadsorption distances d of PEN and PFP, whereas the deviation of thecoherent fractions from unity, with fH ) 0.55 for PEN/C1s and fH ) 0.41for PFP/C1s and PFP/F1s, is related to disorder in the adsorbate system.(b) Schematic conformation of PEN (top) and PFP (bottom) on Cu(111)derived from XSW measurements, indicating the different average positionsof F and C atoms resulting in the intramolecular dipole of µ ≈ 0.53 D. Thelong molecular axis is perpendicular to the page plane. Note that themolecule/metal and intramolecular length scales are different.

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reside above the aromatic core at a distance of (3.08 ( 0.04) Åfrom the Cu surface. This distortion of the PFP molecule allowsestimating the lower limit of the PFP-Cu bonding energy bycomparing the total energies of a molecule in its coplanar andits measured distorted conformation. DFT calculations onisolated, fully geometry-optimized (i.e., planar) molecules, andmolecules optimized under the constraint of the F-atoms being0.1 Å above the plane of the C-atoms yield an energy differenceof ∼0.11 eV. A more detailed analysis of the C1s XSW scanssuggests that also the aromatic core of PFP is not perfectlycoplanar. The core level splitting observed in the C1s spectraallows differentiating between the two different carbon species,that is, those bound to F and those which are not, and acomparison of their relative photoelectron yields in the XSWscan indicates that C atoms bound to F are further away fromthe copper substrate than C atoms not bound to F. Note thatalso PEN chemisorbed on Cu(111) may be nonplanar; however,the splitting of the C1s core level (Figure 1a) is rather smalland thus difficult to resolve in XSW scans.

Compared to PEN, PFP molecules interact weakly with theCu(111) surface leading to a significantly larger bondingdistance. However, the adsorption geometry of PFP is nonplanar,see Figure 3b. Hence, we investigated the consequences of largerbonding distance and the distortion of PFP on the energy levelalignment at the interface to the metal. With UPS we directlyassessed hole injection barriers (HIBs) as the energy differencebetween the low-BE onset of the emission derived from thehighest occupied molecular orbital (HOMO) and the Fermi level(EF) for both molecular monolayer/metal interfaces (Figure 4).Note that for PEN on Cu(119)18 and Cu(110)27 a hybridizationof the PEN HOMO level and metal states was suggested, leadingto new states close to EF; our photoemission data for 2 Å PEN/Cu(111) may also show such a hybrid gap state in the range ofca. 0.5 to 1.0 eV binding energy for θ ) 45° in Figure 4b,however, at very low intensity. The HIBs for PEN (measuredfor the clearly visible peak at 1.1 eV) and PFP (1.35 eV) differonly by 0.25 eV, while their ionization energies differ by 0.85eV (i.e., 5.0 eV for PEN and 5.85 eV for PFP). DFT calculationson isolated, fully geometry-optimized (i.e., planar) PEN and PFPmolecules yield a difference in the HOMO energies of 0.78eV; the adsorption-induced distortion in the PFP molecule doesnot significantly impact its HOMO energy. This shows that thenotion that injection barriers can be easily tuned by changingmolecular IEs is oversimplified, because the organic/metalinteraction can change simultaneously, leading to differentinterface dipoles and thus unexpected injection barriers. Becauseof the discrepancy between differences in HIBs (0.25 eV) andIEs (0.85 eV) of PEN and PFP on Cu(111), significantdifferences in the sample work function (φ) values at monolayercoverage are observed. We find that φ of PEN/Cu(111) is ∆PEN

) 0.9 eV lower than φ of clean Cu(111) (Figure 4a). Thisdecrease of φ upon molecule adsorption is related to the interfacecharge redistribution due to strong chemisorption of PEN onCu.18 Note that the occurrence of the push-back effect ofelectron charge28–30 alone cannot be assumed at this interface,because of the strong chemisorption.

In contrast, φ decreases by only ∆PFP ) 0.35 eV upon theadsorption of PFP on Cu(111) Figure 4. At first approximation,this work function decrease might be explained by a “push-back” of metal surface-electrons. However, in the present casewe also have to consider the dipole of the PFP molecule causedby the adsorption-induced distortion (see Figure 3b). DFTcalculations for the nonplanar PFP yield a dipole moment normalto the molecular plane of µ ≈ 0.53 D. Taking this dipole inducedsample work function change (∆φ) into account via theHelmholtz equation (ε0 vacuum permittivity),

∆φ) ne · µε0 · ε

(1)

and using the surface molecular density n ) 7.8 × 1013 cm-2

obtained from STM (Figure 2b), we obtain a value of ∆φ ≈+0.15 eV for the intramolecular dipole induced work functionincrease (by averaging ∆φ ) +0.16 eV for a relative dielectricconstant ε ) 1, and ∆φ ) +0.13 eV for ε ) 1.2231). Becausethis increase counteracts the lowering of φ, the total contributionrelated to electron push-back is -0.35 eV (∆PFP) - 0.15 eV(intramolecular dipole contribution) ) -0.50 eV (∆PFP,tot).

Noticeably, this value of ∆PFP,tot agrees quantitatively withreported experimental30 and theoretical29 values for physisorbedcyclohexane (C6H12) on Cu(111), where adsorption-induced

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Figure 4. UPS spectra. (a) Secondary electron cutoff and (b) valence regionfor ca. monolayer PEN on Cu(111). (c) Secondary electron cutoff and (d)valence region for ca. monolayer PFP on Cu(111). Photoemission featuresderived from the respective molecular HOMOs are area-filled. θ is the anglebetween the surface normal and the detector. The excitation photon energywas 35 eV.

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lowering of φ is related to Pauli repulsion only. This findingfor PFP on Cu(111) highlights that an adsorption-inducedintramolecular dipole (of an initially nondipolar molecule) mustbe taken into account to derive a complete picture of adsorptionand energy levels. Although details of the adsorption of organicmolecules on metal surfaces can coherently be assessedexperimentally, a reliable theoretical description remains achallenging task.32 It is anticipated that the combination ofappropriate descriptions of organic/metal bonding distances andinterface atomic positions with the approach to assess interfaceenergetics from the interface charge neutrality level5 shouldresult in a refined understanding of interface phenomena thatare of general importance for organic/metal interactions, regard-less of specific interaction strength. In fact, recent extensionsof the induced density of states (IDIS) model, allowing forinclusion of charge transfer, Pauli repulsion, intrinsic moleculardipoles, and interface screening as a function of surface coveragewith molecules, have been successful for describing complexorganic/metal interfaces.33 This model may thus be furtherextended by including adsorption-induced intramolecular dipoles[i.e., as for PFP/Cu(111)], which has not yet been considered.

Following a combined experimental and theoretical approachwe derive important correlations between strength of molecule/metal interaction, average bonding distances, adsorption-inducedmolecular conformation changes leading to intramoleculardipoles, organic/metal interface dipoles, and the resultingunexpected energy level alignment. We find a significantdifference in the average bonding distances for the C atoms of

PEN (2.34 Å) and PFP (2.98 Å) on Cu(111). Furthermore, weshow that even for weak organic/metal interactions [PFP onCu(111)] distortions of the molecule occur, evidenced by the0.1 Å larger bonding distance of F atoms compared to theaverage C position. This results in adsorption-induced intramo-lecular dipoles (∼0.5 D), which influence the interface energet-ics. Adsorption-induced molecular conformational changes haveto be considered as a general phenomenon at organic/metalinterfaces, and will have a significant impact on charge transportacross interfaces, for example, by influencing charge reorganiza-tion energies 21,34 of the molecular monolayer and determiningthe surface potential for subsequently deposited multilayers.21

Furthermore, our results show that changing the molecularionization energy by fluorination does not readily result incorresponding changes of the energy level alignment at organic/metal interfaces according to the Schottky-Mott limit, and othersynthetic strategies should be developed to achieve control overinterfacial energetics.

Acknowledgment. N.K. acknowledges financial support by theEmmy Noether-Program (DFG), G.H. acknowledges financialsupport by the Marie Curie Grant INSANE (EC), and F.S.acknowledges financial support by the DFG and the EPSRC.

Supporting Information Available: XSW confidence levelplot for the atomic positions of PEN and PFP on Cu(111). Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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